1. Field of the Invention
This invention relates generally to optical modulators. More particularly, the present invention relates to electro-absorption modulators. And even more particularly, the present invention relates to a tunable electro-absorption modulator and method of use thereof.
2. Brief Description of the Related Art
A laser transmitter for fiber optic networks must emit signals, at a given stable wavelength, modulated at a desired rate with low chirp and an appropriate power, that are then launched into an optical fiber. Current networks have as many as 100 wavelength channels with one laser""s output devoted to each channel, and each laser having an external modulator. Significantly greater efficiencies could be realized with a laser transmitter and a modulator included on a chip, wherein the modulated laser is capable of being tuned to cover every channel of a system.
Photonic integration can be used to provide a laser transmitter on a chip, as is well understood in the art. While photonic integration is well known in the art, prior art efforts have been focused on the integration of lasers that are not widely tunable. Kobayashi, N.; Noda, A.; Watanabe, T.; Miura, S.; Odagawa, T.; Ogita, S. xe2x80x9c2.5-Gb/s-1200-km transmission of electroabsorption modulator integrated DFB laser with quarter-wavelength-shifted corrugation,xe2x80x9d IEEE Photonics Technology Letters, vol. 11, (no.8), IEEE, August 1999. p.1039-41; Delprat, D.; Ramdane, A.; Silvestre, L.; Ougazzaden, A.; Delorme, F.; Slempkes, S. xe2x80x9c20-Gb/s integrated DBR laser-EA modulator by selective area growth for 1.55-mu m WDM applications,xe2x80x9d IEEE Photonics Technology Letters, vol.9, no.7, IEEE, July 1997. p.898-900. Large tuning ranges make achieving adequate performance of these functional blocks non-obvious with respect to the teachings of the prior art in general, and the prior art related to narrowly tunable devices in particular. What is needed are photonic integration techniques to construct a widely tunable laser apparatus including an integrated modulator that may be effectively tuned according to the selected output wavelength of the laser output.
Laser diodes with integrated electro-absorption modulators (EAM) are becoming important for modem high-speed optical transmission systems. One advantage of the modulator is the higher bandwidth compared to directly modulated lasers. Another is the lower chirp of the modulated signal.
An electrical field is applied across the waveguide of the modulator to change the absorption characteristics of the semiconductor material. Normally the waveguide is embedded in a pin-junction of semiconductor material to apply the field. The waveguide itself is either undoped or slightly p- or n-type doped. If the semiconductor material is reasonably thick the Franz-Keldysh effect applies. In the case of quantum well material, the Quantum Confined Stark effect causes the change in the absorption behavior. In both cases, the electrical field causes an increased absorption for wavelengths below the bandgap energy. This effect is employed in an electro-absorption modulator. The amount of absorption increase depends on the applied electric field strength and the energy separation of the incoming light to the bandgap energy of the semiconductor.
Due to the coupling of the real part of the refractive index to the imaginary part through the Kramers-Kronig Relation also the refractive index of the semiconductor changes when an electrical field is applied. This causes chirp to the light passing through the modulator. The amount of chirp also depends on the applied electrical field strength and the energy separation of the incoming light to the bandgap energy of the semiconductor. Normally, the chirp of the modulator is lower than directly modulated lasers and can even be negative.
In general, the wavelength of the modulator is adjusted to the lasing wavelength by choosing the right composition of the modulator waveguide. The adjustment of the lasing light to the bandgap energy of the modulator determines the absorption of the modulator at zero bias, the absorption as function of the applied reverse bias voltage (and the extinction ratio if an Rf signal is added to the bias voltage) [1], and the chirp of the modulated signal introduced by the associated change in refractive index [2]. For optimum performance this adjustment needs to be very precise. For relaxed tolerances in the fabrication process it would be advantageous to adjust the bandgap energy of the modulator after fabrication.
In the case of tunable lasers the lasing light wavelength is intended to change, but the modulator waveguide bandgap energy can only be adjusted to a fixed wavelength. Therefore any adjustment of the modulator wavelength to the laser wavelength can only offer a compromise. Although the absorption can be adjusted by adjusting the bias voltage, the amount of absorption change with applied voltage varies. Thus, the applied Rf signal magnitude would need to be adjusted. Additionally, the amount of chirp introduced into the signal cannot be controlled by simply adjusting the bias voltage or the Rf signal, because the chirp depends on the wavelength relative to the bandgap energy of the modulator.
Therefore, what is required in the art is a modulator that may be adjustably configured to substantially maximize the absorption of varying wavelengths of coherent light while minimizing chirp and other unwanted effects on the light.
Accordingly, an object of the present invention is to provide a tunable electro-absorption modulator that provides for wide tuning.
Another object of the present invention is to provide a diode laser assembly having a tunable electro-absorption modulator formed integrally therewith.
A further object of the present invention is to provide a widely tunable diode laser assembly with an integrated modulator.
Yet another object of the present invention is to provide a diode laser assembly including a tunable electro-absorption modulator with the elements fabricated on a single wafer by common process steps.
Still another object of the present invention enables the wavelength of the modulator in accordance with the present invention to be adjusted independently of any other optical devices on the same substrate or chip, in particular, a laser diode or a tunable laser diode. This enables the optimization of absorption and/or chirp of the modulator. The change of absorption with voltage relates to the Rf extinction ratio, which can then also be adjusted.
Another object of the present invention is to provide a modulator, the bandgap wavelength adjustment of which enables optimization of transmission and chirp of the modulator.
And still another object of the present invention is to provide a modulator portion that may be tuned to more effectively modulate one of a plurality of wavelengths of output light, received at the modulator from the laser resonator portion.
These and other objects of the present invention are achieved in a laser assembly that includes an epitaxial structure formed on a substrate. A laser resonator portion and a modulator portion are formed in the epitaxial structure.
The present invention will be more clearly understood with reference to the accompanying drawings and the following detailed description, in which like reference numerals refer to like parts and where: